Electrophotographic method of forming relief images

ABSTRACT

Producing a resist by forming an electrostatic latent image on a photoconductive insulating layer containing a resin binder on a base, developing the latent image with a granular toner containing a substance miscible with the resin binder, permeating the miscible substance contained in the toner into the photoconductive insulating layer, applying a solvent capable of dissolving the miscible substance in the binder and yet incapable of dissolving the binder and removing the portion of the photoconductive insulating layer corresponding to those portions wherein the miscible substance has permeated.

United States Patent ELECTROPHOTOGRAPHIC METHOD OF FORMING RELIEF IMAGES 14 Claims, 5 Drawing Figs.

US. Cl 96/1, 252/62.1,117/17.5

Int. Cl 603g 13/22 Field of Search .l 96/ 1 [56] References Cited UNITED STATES PATENTS 3,428,453 2/1969 Honjo 96/1 3,406,061 10/1968 Metcalfe et 211.. 96/1 3,305,359 2/1967 Delmont 96/1 3,121,009 9/1964 Giaimo 96/1 2,857,271 10/1958 Sugarman 96/1 Primary Examiner-George F. Lesmes Assistant Examiner-John C. Cooper, 111

Attorney-Sughrue, Rothwell, Mion, Zinn and MacPeak ABSTRACT: Producing a resist by forming an electrostatic latent image on a photoconductive insulating layer containing a resin binder on a base, developing the latent image with a granular toner containing a substance miscible with the resin binder, permeating the miscible substance contained in the toner into the photoconductive insulating layer, applying a solvent capable of dissolving the miscible substance in the binder and yet incapable of dissolving the binder and removing the portion of the photoconductive insulating layer corresponding to those portions wherein the miscible substance has permeated.

PATENIEUIIEBZBIBYI 7 3,530,728

FIG. I

' FIGZ INVENTORS YASUO TAMAI SATORU HONJO ELECTROPHO'IOGRAPHIC METHOD OF FORMING RELIEF IMAGES BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a novel process of electrophotography and more particularly, it relates to a process for preparing a resist.

2. Description of the Prior Art Numerous methods for electrophotographically obtaining an etch resist have been published. Among them, particularly well known, is a method which comprises forming a toner image on a photoconductive coating provided on a metallic base (said coating comprising a finely divided photoconductor and a hardenable binder, and said toner containing a catalyst promoting the hardening reaction of said binder) causing said toner to permeate sufficiently into the photoconductive coating through the action of either heat or a solvent, then maintaining the metal plate under conditions capable of accelerating the hardening of the binder, and, after the binder has been sufficiently hardened, dissolving the unhardened portion of the photoconductive coating to expose the metallic substrate thereunder.

One disadvantage of this method resides in the fact that it is not suitable for applications wherein only a small portion in a large area is to be etched. For example, this method is not suitable when a plate is desired which has only the letter portions indented on a flat metallic plate. To satisfy such a requirement by the above method, the toner must be deposited onto the entire surface of the background portion except the letter portion. Such a manner of development is not merely difficult to accomplish in accordance with currently available technical standards, but also it is wasteful because it is necessary to consume a large amount of toner for the corrosion of a very small portion. In addition, it is necessary to allow the hardenable resin to sufficiently react, and the operation involved is time-consuming and requires a relatively high temperature.

In obtaining a resist of a form permitting the letter portion to be readily etched by means other than the electrophotographic process, there has been adopted a light-hardening resini.e., photoresist. In this case, a metallic plate to be etched is coated with such a resin, and the plate is exposed to an optical image-containing abundant ultraviolet rays usually through contact exposure-so as to permit the resin in the exposed portion to be hardened. The desired resist can then be obtained by washing away the resin in the unexposed portion.

However, photo resists available currently have a fairly low sensitivity, and therefore require long exposure to ultraviolet rays. Moreover, the use of this process necessitates a vacuum printing frame to assure an intimate contact.

SUMMARY OF THE INVENTION The process of the present invention provides a novel method for forming resists, and also provides a process which meets all of the objects of the invention recited hereinafter.

The process basically comprises the following steps:

1. forming an electrostatic latent image on an electrophotographic light sensitive material comprising a photoconductive insulating layer containing a resin binder on a base;

2. developing the latent image with a granular toner containing a substance miscible with said resin binder;

3. permitting the substance miscible with the resin binder which is present in the resultant toner image to sufficiently permeate into the photoconductive insulating layer so as to permit said substance to dissolve into said binder component;

4. applying a solvent capable of dissolving the substance which is miscible with the resin binder, but incapable of dissolving the resin binder, thereby dissolving said substance; and

5. removing the photoconductive insulating layer in the portion corresponding to the areas wherein the aforementioned miscible substance has permeated.

Accordingly, one object of this invention is to provide a resist which permits letter portions and image portions to be easily corrected by a simple method.

Another object of this invention is to provide a novel process of recording an image.

Still another object of this invention consists in providing a process for the preparation of a negative transparency, the image area of which permits light to be transmitted better than the nonimage area thereof.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic cross section of a photoconductive insulating layer carrying an electrostatic latent image.

FIG. 2 is a schematic cross section of an electrophotographic light sensitive material in which the latent image is developed with a toner.

FIG. 3 is a cross-sectional view of the material of FIG. 2 wherein said toner image has permeated into the photoconductive insulating layer 2.

FIG. 4 is a cross-sectional view of the material of FIG. 3 wherein said permeated area of the photoconductive insulating layer 2 has been removed.

FIG. 5 is a cross-sectional view of the material of FIG. 4 wherein the exposed areas of the base have been etched.

DESCRIPTION OF PREFERRED EMBODIMENTS The invention concerns a process which is characterized by disposing on a suitable base a photoconductive insulating layer containing a resin binder component, forming on said insulating layer an electrostatic latent image by any one of the ordinary electrophotographic techniques, developing said latent image with a granular toner containing a substance miscible with said resin binder, causing said substance present in the resultant toner image to permeate sufficiently into the photoconductive insulating layer, permitting said substance to mix sufficiently into the binder component, and subsequently applying a solvent capable of dissolving said substance but incapable of dissolving said resin binder, whereby the aforementioned substance will be dissolved and the photoconductive insulating layer corresponding to the image portion will be removed at the same time.

The present invention will now be described in further detail with reference to the drawings.

All the drawings show schematically cross section of photosensitive materials.

Referring to FIG. I, a photoconductive insulating layer 2 is coated on a base 1. The electrostatic latent image is shown by an electrically charged area 3 and a discharged area 4. Depending on the purpose, the base 1 may be in various forms. It is necessary that the surface held in contact with the layer 2 possess a suitable level of electroconductivity. The allowable electroconductivity ranges from the general level of metallic conductivity to the level of resistance offered by ordinary paper, namely 10 to 10' ohm (surface resistance exhibited by a square surface). When the base I is made of a metal, the etching of the metallic base will be effected according to the present method. Consequently, it is desirable that the layer 2 withstand the etching solution.

For the preparation of a transparency, an optically transparent film may be used, such as glass, plastic films which are provided with an electrically conductive coating such as NESA coating comprising tin oxide. The electroconductive layer is disposed on the side in contact with the layer 2.

Strictly speaking, in the case of a metallic base, the dark attenuation characteristics of the photoconductive coating is improved by sandwiching a very thin insulating barrier layer between the photoconductive coating and the conductive base or layer. To be more strict, the base should have a conductive laterally continuous layer at least as a part of the whole thickness of the base I, sufficiently close to the photoconductive coating 2.

As an example of a special application, a gelatin matrix can be obtained by providing on the surface of the base 1 a hydrophilic layer, such as of gelatin or casein, which absorbs an aqueous dyestuff easily. This method will be described later.

The photoconductive insulating layer used includes a layer composed of inorganic photoconductive substance, such as ZnO or CdS in powdered form and a resin binder, a layer composed of an organic photoconductive substance and a resin binder, and a layer using an organic photoconductive resin.

In all cases, it is necessary that the layer contain a resin component. Resinous materials which are suitable are those that are not cured, and which can be, if desired, mixed with suitable substance, such as plasticizers. For the photosensitive layer of ZnO, for example, it is especially desirable to use polymethylmethacrylate, polyacrylate, vinylchloridevinyl acetate copolymer, polystyrene, and styrene copolymer, etc.

FIG. 1 illustrates such a photoconductive insulating layer 2 on which is formed an electrostatic latent image by any one of the numerous methods known in the field of electrophotography. As for the methods for forming such electrostatic latent images, typical ones are Carson s process, whereby the layer is uniformly charged in a dark place and then exposed to an image, Kalmans process whereby charging is effected after the exposure to the image, xerothermography and thermoxerography, whereby the exposure is effected thermally, a process based on PIP, and the like.

FIG. 2 shows the manner in which the latent image is developed by a toner 5. The drawing represents the case in which a toner possessed of a positive charge is applied to a negatively charged latent image. This is a case wherein attraction development takes place by Coulomb attraction. It is permissible, of course, to resort to repulsion development using a toner which has the same polarity of charge as the latent image. There is no limitation to the charge sign of the latent image.

Although a toner 5 consisting of a finely divided organic compound which is solid at normal room temperature is most commonly utilized, it is also possible to use a finely divided liquid plasticizer. Illustrative of such compounds, are solid compounds which include triphenylphosphate, chlorinated polyphenyl, tricyclohexylcitrate, 2-butoxyethyl pelargonate, dicylohexyl phthalate, diphenyl phthalate, diethoxyethyl phthalate, 1,2-propylene glycol monostearate, glycerol monostearate, sucrose octacetate, 0- or ptoluenesulfonamide, N-ethyl-p-toluenesulfonamide, N-cyclo-hexyl-ptoluenesulfonamide, and the like. Suitable resinous materials include phenol-formaldehyde resin, maleic acid resin, rosin, Shellac, polyvinyl acetate, epoxy resin, polyketone resin, coumarone-indene resin, and petroleum-base terpene resins, etc.

Compounds of liquid form cited above have a rather polar nature so that they will not dissolve in nonpolar organic solvents, since such liquid compounds are preferably used as fine droplets dispersed in a nonpolar organic liquid which cannot dissolve said compounds. It should be noted that many commercially available liquid plasticizers are partly or completely soluble in common nonpolar organic solvents, making it impossible to prepare a stable dispersion with such solvents. Most plasticizers available on the market are soluble (at least partially) in a wide range of solvents and therefore fail to give sufficient softening.

The solid compounds cited above may be used independently or as a combination of two or more members for use as the toner. In the combined state, the entire toner must be of such a structure as will be dissolved sufficiently in one solvent. Besides, the composition of the toner is determined by taking into account the composition of the resin in the photosensitive material. The toner used must be of such a kind that it shows compatibility with the resin and yet has a difference in dissolution characteristics from the resin.

FIG. 3 illustrates the manner in which the toner image obtained is caused, through a suitable operation to penneate into the interior of the layer 2. To effect such permeation, a solvent for the toner can be sprayed uniformly thereon, the toner image can be exposed to an atmosphere saturated with the vapor of the solvent, or the toner image can be heated to a temperature above the melting point of the toner.

In the case of a photoconductive insulating layer composed of a pulverized inorganic substance and a hinder, the layer is often porous, and therefore permits sufficiently rapid permeation of the toner through heating or a similar treatment. Considering subsequent procedures, however, it is desirable that the toner not only permeate into the layer macroscopically, but also dissolve in the binder phase.

When a solvent capable of dissolving the toner, but not the resin component in the layer, is applied to the recording material carrying the toner image mixed in the layer as shown in FIG. 3, the area 7 of the layer is readily removed from the base 1 leaving the imagewise exposed base 8. What actually takes place at this stage is that only the toner within the layer undergoes dissolution. If the amount of toner is sufficient, toner dissolution makes the total permeated recording layer very fragile, so that the layer in the image area can be easily removed by weak mechanical force.

Actually, the combination of materials is determined by taking into account the condition that the resin and the toner have a different solubility and yet are miscible with each other. The resin used in the electrophotographic layer must have a high insulating property. Therefore, a resin which is strongly hydrophilic is not preferred for this purpose. Generally, those resins which are soluble in ketones, esters and aromatic solvents are extensively used. In general, toner materials which are soluble in polar organic solvents such as lower alcohols, or in extremely nonpolar solvents such as aliphatic hydrocarbons are preferred. The substances for the toner cited above are generally soluble in a very wide range of solvents, and therefore satisfy this requirement.

It is more advantageous if the resin used is insoluble in some kinds of ketone and esters.

Specific cases will be discussed in the preferred embodiments given hereafter.

Images which are produced according to the present invention may be utilized in the following manner:

i. If the base is made of a transparent sheet material, then the plate can be used directly as a negative slide for the purpose of projection or printing. Particularly, in a plate using a ZnO resin layer, a photosensitive layer only a few microns in thickness shows a transmitting optical density in excess of two to light of a wavelength shorter than 400 my" Therefore, such a plate can be utilized as an original plate to be printed by ultraviolet rays. Such a plate is difiicult to prepare electrophotographically, and the method of the present invention can hardly be substituted by any other method.

ii. In the case of a photosensitive layer formed on a metallic base, the base of the image portion can be selectively etched by ordinary etching processes. It is necessary to select a composition capable of withstanding such corrosion for the photosensitive layer. It is permissible to prepare the photosensitive layer in the condition shown in FIG. 4 and then to treat it so as to strengthen its resistance to corrosion. The etched base is illustrated in FIG. 5.

iii. A plurality of dye prints can be obtained by forming a layer of a substance such as gelatin, casein, albumin, or glue (which readily absorbs water-soluble dyes) under the photosensitive layer (the surface of 1), exposing the image portion, then dipping the plate into a bath of water soluble paint so as to absorb the dye, and pressing the plate against a suitable dye-receiving material. In a similar manner, a multicolor image can be obtained. This method may be considered as one route of preparing a dye transfer matrix by electrophotography.

EXAMPLE I A photoconductive layer about microns thick composed of 100 parts of photoconductive zinc oxide and parts of styrene-butadiene copolymer was coated (*(applied dissolved in 50 parts of toluene)) onto the metallized side of a polyethylene terephthalate film having a thin aluminum layer vacuum deposited thereon to prepare a photoconductive recording material. The layer was charged negatively in darkness, exposed to a positive line image to form an electrostatic latent image thereon. A positive toner image was obtained by cascading the layer surface with a cascade developer composed of 100 parts of glass beads coated with nitrocellulose and 1 part of finely divided triphenylphosphate. Since triphenylphosphate holds a positive electric charge, attraction development took place. The developed recording material was exposed to the vapors of trichloroethylene for a short period to fix the image. Then, when the photosensitive layer was lightly wiped with a pad of gauze impregnated with a solution consisting of parts of water and 70 parts of acetone, triphenylphosphate on the image portion became dissolved, and the corresponding portion of the photosensitive layer was removed completely, with the result that the aluminum layer was exposed in the portion corresponding to the image. The styrene-butadiene copolymer used as the binder was insoluble in acetone. When the plate was dipped in a dilute aqueous solution of NaGl-i, the exposed aluminum at once dissolved so that the line image portion became transparent. The plate was quickly washed in water, dried, and then used as a transparency to yield satisfactory results.

EXAMPLE II This example was identical with example I, except that the toner used in the development was composed of 10 parts of triphenylphosphate and 2 parts of black toner for use in Xerox 914 copying machines (available on the market) and that the fixation of image was effected by infrared heating. The plate produced substantially the same results as in example i.

EXAMPLE III A photoconductive insulating layer composed of 100 parts of photoconductive zinc oxide, 20 parts of polymethylmethacrylate,* (*(polymerization degree equal to 500)) and 10 parts of alkyd resin modified with acryl ester was provided on a plate of zinc in such manner as would give a dry thickness** ((ZnO and binder applied in 60 parts of toluene)) of about 10 microns. The layer was charged negatively and exposed through a positive image to light to yield a latent image thereon.

The latent image was developed by a toner which was obtained by pulverizing a molten mixture consisting of 5 parts of decyclohexylphthalate and 5 parts of an alcohol-soluble rosinmodified phenol resin. The toner possessed a positive electric charge and therefore effected the attraction development to give a positive toner image.

When the temperature of zinc plate carrying the image was raised to about 100 C., the image was observed to be absorbed into the photosensitive layer. After the plate was cooled, the layer surface was wiped with a mixture consisting of 50 parts of methanol and 50 parts of ethanol. Consequently, the image portion was observed to be dissolved out.

The layer surface was then uniformly sprayed with a toluene solution* (*(5 toluene solution)) of cobalt naphthenate and heated at 120 C. for a short period to harden the alkyd resin present in the photosensitive layer with a view to strengthening the resistance thereof to corrosion. Next, the unit was immersed in an aqueous solution of ferrous chloride etchant so that the portion stripped of the photosensitive layer was corroded to produce an intaglio.

EXAMPLE iv A layer composed of parts of photoconductive zinc oxide and 25 parts of vinyl chloride-vinyl acetate copolymer was provided on the coated surface of a waterproof paper which was undercoated with gelatin to a thickness of 2 11.. The thickness of the photosensitive layer was about 5 ,u.. Using the same developer as in example Ii, a latent image corresponding to the negative original was developed. The image was caused to impregnate into the layer in the same manner as in example II, and the photosensitive layer in the image portion was removed by wiping with methanol. When the paper was immersed for a short time in the aqueous solution of pronase (protease), the exposed portion of gelatin layer was dissolved out. The paper was quickly removed, dried, and then stripped of the photosensitive layer with toluene to obtain a positive gelatin matrix.

In view of the examples provided above, the following information is offered to amplify upon the present invention at several points.

Generally, it has been found that ratio of binder to photoconductor (by weight) should be within the range l:25l:3, with the most preferred range being 1212-1 :4.

Further, the amount of toner (equivalent to plasticizer in its general purpose), the amount of plasticizer, is generally from about one-fifth to five times (by volume) the amount of binder used in the photoconductive insulating layer. if the amount of plasticizeris less than 20 percent the volume of the binder, the insulating layer cannot be removed upon dissolving by the plasticizer. If the amount of plasticizer is greater than five times by volume the amount of binder, image quality suffers.

In general, the thickness of the photoconductive layer will vary from a few microns up to about 20 microns as a most preferred range. The volume of the binder in the layer corresponds, of course, to the thickness thereof, generally about l-lO microns. Of course, for this range the amount of plasticizer will be 0.2-40 grams per square meter.

To further amplify upon the binder somewhat, when the binder is unhardened, if it is thermosetting or thermoplastic, the insulating'layer using the binder can be removed by dissolving of the toner. Examples of thermoplastic binders utilizable in the present invention are polyvinyl acetate, vinyl acetate-crotonic copolymer, polyvinyl chloride, vinyl chloride-vinyl acetate copolymer, polyalkyl acrylate, polyalkyl methacrylate, vinyl copolymers of alkylacrylate or alkyl methacrylates, polystyrene, and a styrene-butadiene copolymers. Examples of thermosetting binders useful are epoxy resins, epoxy ester resins, silicone resins, alkyd resins, alkyd resins modified with vinyl monomers, and mixtures thereof.

The plasticizer (toner material) must have the capability of dissolving substantially completely in some solvent, and yet it must have some capatibility with the binder resin. By compatibility it is meant that the plasticizer must illustrate a sufficient dissolution to achieve the results of this invention, but it need not illustrate complete dissolution.

To further define the binder resin-toner solvent relationship, it is required that the binder resin be completely insolvent in the toner solvent.

In general, by the language sufficient permeation," the toner will, in most cases, permeate entirely to reach the base 1. However, even if the toner does not permeate to completely reach the base 1 the image can be formed.

With respect to describing the resin as illustrating a high insulating property, this means that the resin must have a higher volume specific resistance than that of the photoconductor utilized in the dark. Generally, this means that the resin most preferably illustrates a volume specific resistance greater than 10 ohm-cm.

Since the combination of the binder and the toner utilized is important to the present invention, the following combinations are offered to illustrate those which are acceptable in the present invention:

COMBINATION I When the toner is soluble in a polar solvent, the binder should be insoluble in the polar solvent. With reference to the resins set out above, this would apply to the thermosetting or the thermoplastic resins with the exception of the vinyl acetate, vinyl acetate-crotonic acid copolymer. The toner solvent would be, for instance, a polar solvent such as methanol.

COMBINATION 11 When the toner is soluble in a nonpolar solvent, such as chlorinated polyphenyl, the binder is insoluble in a nonpolar solvent. Such binders would be polyvinyl acetate and a vinyl acetate-vinylchloride copolymer. The toner solvent should, in this case, be a nonpolar solvent such as cyclohexane, kerosene, or decalin.

COMBINATION 111 Once the toner is soluble in acetone, the binder must be insoluble in acetone. For instance, an acceptable binder in this case is a styrene-butadiene copolymer, such as that utilized in example I, wherein the styrene-butadiene copolymer (styrenezbutadiene 85:15) was Pliolite S- D (trade name) manufactured by the Goodyear Company Ltd. In this case, the toner solvent would be most preferably acetone.

We claim:

1. An electrophotographic process for producing a resist which comprises:

a. electrophotographically forming an electrostatic image on the insulating layer of a photographic light-sensitive material comprising a photoconductive insulating layer containing a resin binder selected from the group consisting of thermoplastic resins and uncured thermosetting resins on an electrically conductive base;

b. developing said latent image with a thermoplastic granular toner containing an organic plasticizer compound compatible with said resin binder;

c. permitting said plasticizer compound present in the resultant toner image to permeate into the photoconductive insulating binder layer d. applying a solvent capable of dissolving the aforementioned plasticizer compound which is incapable of significantly dissolving the resin binder, thereby dissolving said aforementioned plasticizer compound, and

e. removing the photoconductive insulating layer in the portion corresponding to the areas wherein the aforementioned plasticizer compound has permeated and has been dissolved by said solvent, said surface electroconductivity of the base being no greater than ohms-cm.

2. The electrophotographic process claimed in claim 1, wherein said resin binder is a styrene-butadiene copolymer, said granular toner is soluble in acetone and said solvent is acetone.

3. The electrophotographic process claimed in claim 1, wherein said resin binder is insoluble in methanol and said granular toner is soluble in methanol.

4. The electrophotographic process claimed in claim 1, wherein said photoconductive layer comprises ZnO and a resin binder selected from the group consisting of polymethyl methacrylate, polyacrylates, vinyl chloride-vinyl acetate copolymer, polystyrene and a styrene based copolymer.

5. The electrophotographic process claimed in claim 1, wherein said granular toner contains a plasticizer selected from the group consisting of triphenyl phosphate, chlorinated polyphenyl, tricyclohexyl citrate, 2-butoxylethy1 pelargonate, dicyclohexyl phthalate, diphenyl phthalate, diethoxyethyl phthalate, 1,2-propy1ene glycol monostearate, glycerol monostearate, sucrose octacetate, o-toluenesulfonamide, ptoluenesulfonamide, N-ethyl-p-to1uenesulfonamide, and N- cyclohexyl-p-toluenesulfonamide.

6. The electrophotographic process claimed in claim 1, wherein said photoconductive layer comprises CdS and a resin binder selected from the group consistingof pol methyl methacrylate, polyacrylates, vinylchloride-vmy acetate copolymer, polystyrene and a styrene based copolymer.

7. The electrophotographic process claimed in claim 1, wherein said thermoplastic binder is a member selected from the group consisting of polyvinylacetate, vinylacetate-crotonic copolymer, polyvinylchloride, vinyl chloride-vinylacetate copolymer, polyalkyl acrylate, polyalkyl methacrylate, vinyl copolymers of alkylacrylate or alkyl methacrylates, polystyrene, and a styrene-butadiene copolymer.

8. The electrophotographic process claimed in claim 1, wherein the thermosetting resin is a member selected from the group consisting of epoxy resins, epoxy ester resins, silicone resins, alkyd resins, alkyd resins modified with vinyl monomers and mixtures thereof.

9. The electrophotographic process claimed in claim 1, wherein the ratio of binder to photoconductor, by weight, is from 1:25 to 1:3.

10. The electrophotographic process claimed in claim 9, wherein the ratio of binder to photoconductor, by weight, ranges from l:l2 to 1:4.

11. The electrophotographic process claimed in claim 1, wherein the amount of plasticizer ranges from about one-fifth to five times, by volume, the amount of binder employed in the photoconductive insulating layer.

12. The electrophotographic process claimed in claim 1, wherein the thickness of the photoconductive layer ranges from 1 to 20 microns.

13. The electrophotographic process claimed in claim 12, wherein the thickness of the photoconductive layer ranges from 1 to 10 microns.

14. The electrophotographic process of claim 1, wherein said granular toner contains a binder for said plasticizer, said binder being a member selected from the group consisting of phenol-formaldehyde resin, maleic acid resin, rosin, shellac, polyvinyl acetate, epoxy resin, polyketone resin, coumarone, indene resin, and petroleum-based terpene resins. 

2. The electrophotographic process claimed in claim 1, wherein said resin binder is a styrene-butadiene copolymer, said granular toner is soluble in acetone and said solvent is acetone.
 3. The electrophotographic process claimed in claim 1, wherein said resin binder is insoluble in methanol and said granular toner is sOluble in methanol.
 4. The electrophotographic process claimed in claim 1, wherein said photoconductive layer comprises ZnO and a resin binder selected from the group consisting of polymethyl methacrylate, polyacrylates, vinyl chloride - vinyl acetate copolymer, polystyrene and a styrene based copolymer.
 5. The electrophotographic process claimed in claim 1, wherein said granular toner contains a plasticizer selected from the group consisting of triphenyl phosphate, chlorinated polyphenyl, tricyclohexyl citrate, 2-butoxylethyl pelargonate, dicyclohexyl phthalate, diphenyl phthalate, diethoxyethyl phthalate, 1,2-propylene glycol monostearate, glycerol monostearate, sucrose octacetate, o-toluenesulfonamide, p-toluenesulfonamide, N-ethyl-p-toluenesulfonamide, and N-cyclohexyl-p-toluenesulfonamide.
 6. The electrophotographic process claimed in claim 1, wherein said photoconductive layer comprises CdS and a resin binder selected from the group consisting of polymethyl methacrylate, polyacrylates, vinylchloride-vinylacetate copolymer, polystyrene and a styrene based copolymer.
 7. The electrophotographic process claimed in claim 1, wherein said thermoplastic binder is a member selected from the group consisting of polyvinylacetate, vinylacetate-crotonic copolymer, polyvinylchloride, vinyl chloride-vinylacetate copolymer, polyalkyl acrylate, polyalkyl methacrylate, vinyl copolymers of alkylacrylate or alkyl methacrylates, polystyrene, and a styrene-butadiene copolymer.
 8. The electrophotographic process claimed in claim 1, wherein the thermosetting resin is a member selected from the group consisting of epoxy resins, epoxy ester resins, silicone resins, alkyd resins, alkyd resins modified with vinyl monomers and mixtures thereof.
 9. The electrophotographic process claimed in claim 1, wherein the ratio of binder to photoconductor, by weight, is from 1:25 to 1:3.
 10. The electrophotographic process claimed in claim 9, wherein the ratio of binder to photoconductor, by weight, ranges from 1: 12 to 1:4.
 11. The electrophotographic process claimed in claim 1, wherein the amount of plasticizer ranges from about one-fifth to five times, by volume, the amount of binder employed in the photoconductive insulating layer.
 12. The electrophotographic process claimed in claim 1, wherein the thickness of the photoconductive layer ranges from 1 to 20 microns.
 13. The electrophotographic process claimed in claim 12, wherein the thickness of the photoconductive layer ranges from 1 to 10 microns.
 14. The electrophotographic process of claim 1, wherein said granular toner contains a binder for said plasticizer, said binder being a member selected from the group consisting of phenol-formaldehyde resin, maleic acid resin, rosin, shellac, polyvinyl acetate, epoxy resin, polyketone resin, coumarone, indene resin, and petroleum-based terpene resins. 